1. Field of the Invention
This invention pertains to the conversion of picture information on a motion picture film into a video signal and, more particularly, to a film scanning apparatus commonly known as a telecine scanner, which is used for playback of a motion picture film for television production and programming.
2. Background Art
In a motion picture, the original scene is exposed upon negative photographic film, producing three interrelated dye records of the original scene. Cyan, magenta, and yellow dyes are typically used in a color negative film, and their amounts correspond respectively to the red, green, and blue information in the scene. Each dye absorbs light preferentially in a different region of the spectrum with peak absorption in the desired red, green, or blue regions. Typical yellow, magenta, and cyan dyes preferentially absorb bands of the spectrum from about 380 to 490 nm ("blue" light), 490 to 580 nm ("green" light), and 580 to 740 nm ("red" light), respectively. The spectral densities associated with such absorptions are designed for compatibility with the duplication and editing steps normally used in a motion picture film production. Through a series of duplicating steps the negative film original is used to produce a positive print film. The spectral densities of the print film dyes represent a positive image of the original scene and are designed for direct viewing through use of a motion picture projector.
Motion picture print film has been traditionally preferred for telecine scanning because positive prints, besides being readily available, are already color balanced for direct viewing and require fewer color corrections than a negative film. However, the making of a positive film print from the original negative film requires at least one extra processing step, which results in some degradation of the image as well as color saturation of the resulting print relative to the negative. A negative film in a telecine scanner can handle the tone scale from highlights to shadows with less distortion than print film, thus resulting in better color reproduction. All things being considered, therefore, it is desirable to be able to use both positive and negative films in a telecine scanner without undue inconvenience.
Ideally, a telecine scanner should measure the optical modulation caused by each dye in order to correctly estimate the red, green, and blue content of the original scene and produce a subjectively pleasing image. These measurements are difficult to obtain because the yellow, magenta, and cyan dyes used in film not only absorb light in the desired blue, green, and red bands of the spectrum, respectively, but each also absorbs in more than just the desired band. To eliminate the effects of the unwanted dye absorptions in the film and to improve color/tone scale quality, the ideal solution would be to have monochromatic telecine sensitivities at the three wavelengths corresponding to the peak absorptions of the three dyes. However, the narrower the spectral response of each color channel, the lesser the system efficiency or sensitivity. In order to waste as little as possible of the available light, dichroic beamsplitters and trimming filters are usually used to divide the light into three bands with slight overlaps in the 490 nm and 580 nm regions. (See "Color Bars on Film for Setting Up Telecines," by R. W. G. Hunt, SMPTE Journal, February, 1978, vol. 87, pgs. 78-81). Nonetheless, wide spectral responses result in the aforementioned color crosstalk (e.g., by measuring magenta dye in the cyan measurement channel), and further result in a system gamma that is at least to some extent a function of density, rather than color. Telecine spectral sensitivity, thus, is necessarily a compromise between sensitivity and color/tone scale quality.
U.S. Pat. No. 3,944,739 discloses a flying spot television scanner that operates in the aforementioned way with beam splitters and trimming filters to separate light from a scanned film into red, green, and blue components centered on predetermined wavelengths prescribed by standards accepted for color television. The components are obtained by a red reflecting dichroic mirror arranged to reflect red light through a red filter to a photosensor, and to transmit blue and green light. The transmitted blue portion is reflected by a second dichroic mirror through the series combination of a low pass blue filter and a high pass blue filter to another photosensor. The green light portion is transmitted through the second dichroic mirror and a green filter to a third photosensor. The combination of the dichroic mirrors and the light filters resolve and shape the light transmitted through the film into three components which are respectively red, blue and green.
Conventional telecine scanners have a fixed set of sensitivity peaks that generally correspond to a particular scanning standard or dye set. Despite such singular focus upon a particular standard or motion picture film, unwanted absorptions caused by overlapping spectral densities of the dye set are always present and may be further accentuated if for any reason the scanner sensitivity peaks do not coincide with the peak spectral absorption of the dyes. A matrix circuit is used to process the electrical scanning signals to enable corrections to be made for the effect of such unwanted absorptions and spectral misalignments. If different films, however, are run through the telecine scanner, the different dye sets are analyzed in different ways, and the telecine matrix settings may have to be adjusted. In many cases, the greater spectral misalignments require even more matrixing. Color matrixing, however, has the disadvantage of increasing noise in the resulting output and should be minimized for systems requiring high signal-to-noise performance.